Floating roof storage tanks



Aug. 7, 1951 A. s. Fit-:ILD

FLOATING ROOF STORAGE TANK 3 Sheets-Sheet 1 Filed Jan. ll, 1947 Alexander' Feud E@ N5 Morneri Fig. 2`

Aug. 7, 1951 A, s, FElLD 2,563,013

FLOTING ROOF STORAGE TANK Filed Jan. 1l, 1947 3 Sheebs-Shee 2 lrwen'or' Nexonder 5. Fand l Esq hs MtorneLj'.

ug- 7, 1951 A. s. FEILD 2,563,016

FLOATING ROOF STORAGE TANK Patented Aug. 7, 1951 FLOATING ROOF STORAGE TANKS Alexander S. Feild, Houston, Tex., assignor to Shell Development Company, San Francisco, Calif., a corporation of Delaware Application January 11, 1947, Serial No. 721,629

15 Claims. Cl. 220-26) 1 This invention relates to storage tanks suitable for storing vvolatile liquids, such as gasoline and other petroleum products, of the type having a roof iioating on the stored liquid and vertically movable within the tank shell to change the storage space` as the stored liquid is introduced into or drained from the tank. The instant invention is, more particularly, concerned with the structure between the tank wall and the floating roof provided to seal the annular space around the roof against evaporation losses and for centering the floating roof within the tank shell.

Storage tanks of this type are commonly circular and are provided with circular iioating roofs having diameters lsmaller than the inside diameter of the tank, leaving an annular space between the tank and the floating roof. Friction means, such as a plurality of shoes or runners, supported by the roof. are urged yieldably outwardly against the tank wall. and a flexible septum, made of impregnated fabric, or of natural or synthetic rubber seals the space between the friction means and the roof, this space being variable due to the yieldable arrangement.

Such septa are curved around the roof and the ilexure occurring as the width of the annular space varies is not along straight lines. but along a curve or along a number of short and irregular creases. It is, therefore, not feasible to employ-more durable materials, such as metal, for the septa with Athe constructions now in use. Sealing means containing such septa have been found to be expensive and difcult to maintain and repair because the flexible septa are subject to rapid deterioration due to weathering and the action of corrosive vapors from the stored liquid. Theyare also fire hazards because such fabrics and other ilexible materials are destroyed by re, and a fire at one point on the rim of the roof usually spreads all around the rim.

It is an object of this invention to provide a seal for a oating roof which can be constructed entirely of fire-proof and durable material, such as metal or similarresilient material, although the invention is directed to the arrangement of parts whereby such materials of construction may be utilized, and is not to be limited to the use of such materials. Ancillary to this object, it is a purpose to provide a sealing arrangement between a sealing shoe and the tank comprising durable material, such as metal or plastic, wherein relative motion is made possible by bending the durable material along one or more straight 2 about said lines, either by constructing the seal out of resilient material which will iiex along said hinge line or lines, or by providing vaportight hinges along such lines.

It is a further object of the invention to provide an improved sealing arrangement for floating roof storage tanks which will center the roof Within the tank shell by automatically providing an increased horizontal thrust between the roof and the portion of the tank shell toward which the roof has been displaced, whereby it is not necessary to provide rigid vertical columns Within the tank. A subsidiary obJect is to center the roof Within the tank by means of hydraulic pressure.

A further object is to provide sectional sealing elements for oating roof storage tanks that are readily removable and replaceable individually Without draining the tank of its stored liquid.

Still another object of the invention is to provide sealing elements for floating roof storage tanks that utilize the pressure of vapors generated from volatile stored liquid to urge the sealing shoes against the tank wall. Ancillary to this object, it is an object to provide means for equalizing the pressure of the vapors acting upon diierent sealing elements, thereby obviating inequalities in pressures which would be created, for example, by differences in temperature at different points of the perimeter.

Another object of the invention is to provide a floating roof storage tank in which the force urging the shoes against the tank wall can be easily regulated, and in which excess vapors acting upon the sealing elements can be vented.

A further object of the invention is to provide a sealing element for a floating roof storage tank comprising a sectional shoe pressed against the tank Wall by the combined action of vapors generated by the stored liquid and trapped within a bellows, and by an auxiliary pressure element comprising a bellows filled with a liquid.

Other'objects of the invention will become apparent from the following description, taken together with the drawings, in which:

Fig, 1 is a plan view of a storage tank constructed according to my invention;

Fig. 2 is an enlarged fragmentary sectional view taken on line 2-2 of Fig. 1;

Fig. 3 is a fragmentary horizontal section view taken on line 3-3 of Fig. 2;

Fig. 4 is an exploded perspective view showing hinge lines and permitting hinge-like movement the parts of the sealing element;

Such a sheet is lined by vapor-tight connections, and, by fliing, peri" mits the distance between theV sealing shoe and the floating roof to vary, as is necessary toallow Y for irregularities in the shape of the tank Wal-l-.

Such sheets of durable materialhaveustraisht hinge lines extending preferably tangentially to the outline of the floating roof. To cover the en-A tire periphery oi the roof they are arranged end toend in sections. tfis notnecess'ar-y-to-make-the roofv circular;- in fact; another vimportant y featureA ofY this invention is a polygonal construction of the iloating roof, whereby the str-aight sealing. sections can be mounted at the edges-of therooi. The invention is not, however, limited.y to polygo-A nal roofs; since it -is possible to mount straight sealing sectionson top of a circular roof. Y Hinge-like motion along the straight hinge lines is most conveniently e'ected by making the sealing sheet vof 'resilient or spring-like metal or plastic. The sealing sheet may, howeveigfbe constructed of several-pieces of stiffer material and motion achieved byinstalling hinges at the hinge lines. v

According t one embodiment of thepresent invention, a iioating roof is sealed against thetank bya plurality of sectional sealing elements, each sealing element comprising a sealing shoe and o cured by inverted U-frames 2l.

a compartmented pressure seal provided with, a

sealingsheet corrugated to have straight iexure lines. In the specific embodiment described below. fourteen sectional sealing elements are used to seal the perimeter of the roof, but thisnumber maybe varied.

The sealingvshoe-m'ay .be of conventional forni. The compa-rtmented pressure seal comprises three longitudinalwalls movably joined together so that thetwo outer side walls may be moved apart or. broughtne'arer each other by substantially parallel motion, and arranged toformrtwo compartments: One compartment is trough shaped, and is formed by aside Ywall and the medial wall; thepthercompartment isY inV the forni or" an inverted trough, and is formedl by 'the other side Wall and the medial Wall. Each compartment has itsV ends closed by means of bellows. rIfhe walls areurged apart by means offluids and/or springorsimilar devices, One side wall is at the roofu and thevother .at the sealing shoe, whereby theshoe is(urgedoutwardlyland the space betiveenthe roof and sealing shoe is sealed.

ReferringY to the drawing, the invention is shown applied to a storage tank having a circular side Wall l0. The floating root` comprises a central portion ll, constructed of thin, resilient metal (it being understood that such portion would be constructed of many small sections Vor plates, joined together and may be providedvvith dams, not shown, to reduce Wash of the water when the roof is tilted) and heavier marginal rim angles l2, having upright legs i3. The rim angles l2 give the roof .a polygonal outline, and the upright legs I3 rare joined together to Yprovide a Water-tight rim. Legs i3 Vshould extendV to above the hydrostatic head of the stored liquid to prevent overflow thereof from the annular space between the tank Wall and the roof onto the roof deck.

The sealing elements may be applied to any type of floating roof, including roofs supported by pontoons `floating on the stored liquid, and single-deck or double-deck roofs. The elements have been shown, in the drawings, as applied to Aa single-deck pressure roof, in which a layer oi water, hl, is placed on top of the roof to Weight it to Ycounteract the vaporization of the liquids. Such a roof may be provided IWith a circular stiiieningr angle l5 at the center, within which are mounted a manhole i6, water-supply jvand overflow pipes il and I8 (connected to .a

point outside of the tank by means of a flexible hose beneath the roof, not shown) and an emergency dump valve il for dumping water from the roof into. the tank when the Weight of the WaterA becomes too great .with respect to the pressure ofthe iiuid'stored. Within the tank. The details of these vali/'es and of the water-supply and drainage systems are shown in greater detail in my copending application Ser. No. 721,630, led January 1l, 1947.

Because or" the greater Weight of the stiiened rim angles. iE and the .pressure elements, such a lioating roof would tend to bulgefupvvardly in the center, were not some provision made to balance the loads. 'This balance is eected by providing pontoons over the angle rims, 'se- Fig. e, the height of the pontoon can be adjusted by boltingi the pontoon l'at Aany height .on the frames 2i by passing bolts through the holes 22 of the frame and holes 23. provided in the end flange plates of the ponto'ons.v By lowering the pontoons a greatersubmergencein the Water lll and a correspondingly greater lift on the rim angles i2 is provided. vThepontoon*construction and balancing Vof loads is lmore `fully described and claimed in the aforesaid copending application. 1

To supportthe'roof onthe tank bottom YWhen the tank is drained, "a plurality.oli -legs 24is provided, four 'being shojvvil Within the stffening angle l5 and'one between each pair ofV adjacent pontoons. The' legsiaie vertically adjustable and slide Withina telescopingtiibe (seelFig. '2) esitenodng'thrilgh and sealed tothe IIli angle 12. Rgidljy is insured by`Ilf(")l.l-Ifl",lgv Cha'nllbai's 26 between Yadj acont `trainee? I arid Welding the tubes 25 into holes therethrough.- Theh'eight 'of the legs A2i may be adjusted `'by inserting a pin 2 through a transverse hole in the "tube Y25 and throughfanyone of the jholes 2B inthe'leg. rThe vertical thrus'tlis carriedfpartlylbythe `traine 2l and partly bythe direct ('onnectio hetvveen'the tube 25 and thefrinrangle`l2. 2 agangway, Submited by. rsciaflsillafrbiooh 3f@ *and by 'the upright ieg it ofjj-onegof the fahg'iegrims i2.V "el and'are ladders. Y Y v.

vTurning now tofthe' novel'se'a'ling lements vvith which thisinventionris particularly concerned, each 'sectional sealing element' bcomprises a sealin'gshoe 33 curved, tontag'ainst theftank Wall andihavingitlietop" di bot i edges turnedin- Wardly'to ride .over irl egularitiesiofthe'wall. The ompartmented pressure Aseal includes a' sealing sheet/or plate Biof'resilienft/"metal folded'and shaped as Vfollow/Ls The `outer hcnfzizontal shelf ,A 3.4@ has'the,generalshape:ofa'segmntfof'a circle,

and has "an uptrrfednangeeficientes to et As is seen from 5, the curvature of the sealing shoe. Thesealing shoe is attached to the plate by welding to the ange 34h. At the inner edge of the shelf the plate is folded downwardly to provide the medial Wall 34e. At the bottom two approximately rightangle folds provide a narrow width of iioor 34d. The width of this floor and the width of the shelf 34a are such as to accommodate the bellows (to be described). The inner rising portion 34e, forming the inner wall, has a iiange 34j at its top. It will be seen that a trough or inner compartment A is formed between the inner wall 34e and the medial wall 34e, and that an inverted trough or outer compartment B is formed between the medial 'wall 34e and the inner face lof the sealing shoe 33.

The fold between the outer shelf and the medial wall and at least one (preferably both) of the folds between the medial wall and the inner wall should be straight lines. This permits vthe outer wall or shoe to move away from or towards the inner wall with substantially parallel movement, primarily by flexure at the straight-line folds, some lesser amount of bending also taking place in the outer shelf, medial wall and floor. Such relatively parallel motion is necessary when the roof moves horizontally. Under certain conditions, however, a limited degree or angular motion between the shoe 33 and the roof is required. This would occur, for example, when the roof is tilted by wind or uneven distribution of the water on its surface. It is apparent that the construction described is not restricted to parallel motion but permits also a limited .angular movement.

While I have shown the inner and medial walls to be formed 'of the same piece 34 and the outer wall to be identical with the sealing shoe 33, it is evident that any other arrangement of parts may be employed, ,and that the only essential requirement is that the walls have the relative motion described. Thus, a greater or a smaller number of parts may be employed, for forming the compartment walls, and hinges may be placed at the folds, if desired.

It will be seen that by providing a roof of polygonal instead of circular outline it becomes possible to achieve improved sealing between the tank wall and the iioating roof, since durable materials such as metal can be flexed or hinged only along straight lines.

The ends of the compartments are provided with yieldable end closures, such as corrugated metal bellows 35 and 33. They are welded or otherwise sealed against the walls of the compartments and the shelf and iioor. As is seen from Fig. 2, the bellows 35 extend several inches below the level of the rim angle I2. The inner margin of each bellows 35 is welded to a depending liquid seal plate 31, attached to the bottom of the inner wall 34e. Liquid seal plates 31 from adjacent pressure seals are fitted closely together, so as to prevent the escape of vapors collected against the bottom of the roof into the space between adjacent pressure seals. To assist in closing the gap between such liquid seal plates 31 a short angle piece k38, bent to conform to the angular displacement of adjacent pressure seals, may be mounted beneath the rim seal I2. The outer face of the angle piece 38 abuts the inner faces of the plates 31. The space may also be sealed by other means, such as gaskets or by a tongue-and-groove or chamfered joint between adjacent plates 31.

The pressure seal is mounted on' the roof by bolts 39 engaging an angle 40 permanently aiiixed 62! to the leg I3 of the rim angle I2. A rain shield 4I, constructed of thin, sheet metal is optionally provided to prevent rain water from entering the inner compartment A. The shield is supported by a Z-shaped raingutter 42 mounted on the shelf 34a and extending the length of the shelf, whereby water seeping into the space above the shelf is prevented from iiowing into the compartment A. j Such water, occurring only in minor amounts, discharges into the space between adjacent sealing elements, as will be seen from Fig.

`3. A curved shingle 43 engages the tank wall and leads water to the shed 4I it is constructed of flexible material, such as rubber or fabric, a1- though more durable materials may be employed. The rain shields 4I are constructed in sections corresponding to the sealing elements, there being an overlap between adjacent sections to provide a continuous shield.

A further optional feature of the pressure seal is the vapor pressure equalizer. It comprises an annular pipe 44 provided with a T tting 45 over each outer compartment B, and connected to such compartments by riser pipes 4S and valves 41. The Ts 45 have bolted anges to permit a section of the pipe 44 to be readily removed.

Water -or other liquid is placed into each inner compartment A as shown. This liquid, acting over the entire length of each pressure seal, exerts a `constant `pressure tending to distend compartment A and the bellows 36. This thrust is ,transmitted to the sealing shoe 33 mainly through the outer shelf 34a, and in part by the vapors in the compartment B, as described below. When the roof is fortuitously displaced toward one side of the tank the bellows are compressed, the walls of the compartment A on the side nearest the tank are brought nearer together, and the level of the liquid therein rises. This increased head results in an increased hydraulic pressure against the medial wall and a greater threat on the sealing shoe. Simultaneously, in the pressure seal at the diametrically opposite side, the roof has moved away from the tank wall, with the opposite effects, resulting in a lowering of the liquid level and a decrease in the horizontal thrust. These two effects conspire to impose a centering force on the roof.

The outer compartment B of the pressure seals are filled with vapors generated from the stored liquid. Because, in the construction described herein, these compartments are effectively and reliably sealed against the escape of vapors, a considerable pressure is built up therein, depending upon the volatility and temperature of the liquid. Some vapors are unavoidable formed beneath the oating roof, regardless of the .pressure imposed on the stored liquid by the weighted roof. Suchv vapors are able to seep into the outer compartments B, passing .beneath the i'loor 34d of compartments A, between the liquid seal plates 31 and the angles 38. Such vapors cannot, however, pass into the space C (see Fig. 3) between sealing elements, because they are trapped by the liquid seal plates 31, angles 38, and the downwardly extending bellows 35. Some evaporation lossesare possible from the surface of the stored liquid in the spaces C, which are open to the atmosphere. In a typical tank having e, diameter of 60 feet, each of these spaces is only about l0 sq. inches in area, and the fourteen spaces C around the rim of the roof present a. total evaporation area of less than one square foot. This vis less than 0.04% of the total area of the roof and isnegligible.

The vapors in the outer compartments B urge the wall' 351e and the sealing shoe 33 apart, thereby applying an even pressure against the outer wall throughout its height and length. When the liquid stored is highly volatile and a greater sealing force is required, the increasing pressure in the compartments B automatically results in a greater sealing pressure.

Because the sealing shoe 33 and the inner wall 34e are, in the usual horizontal movement of the roof, constrained to move substantially only in translation or parallel motion, it is evident that the dis-tension of one compartment results in a corresponding distension of the other compartment. But the pressure of the vapors Within compartment E is, under given conditions of volatility and temperature, more or less fixed, so that the increased thrust caused by a rise in the level of the liquid in the compartment A is transmitted largely through the shelf 34a. Thus, when the compartments are compressed to cause a rise in the liquid level in compartment A, the level of the stored liquid in compartment B is at first depressed, with a slight increase in pressure (assuming the valve 4l to be closed). When, however, the liquid is depressed to below the level of the liquid seal plates 3l the gas escapes and no further rise in vapor pressure occurs. Moreover, when the vapor pressure equalizer is installed and the valve 41 is open the gas will escape through the pipes lll and 46 to enter a compartment on the diametrically opposite side of the tank.

The horizontal centering thrust created by the liquid in the compartments A can be regulated by filling them with a greater or smaller amount oi liquid. The pressures in the compartments B are equalized by the vapor pressure equalizer, thereby automatically permitting the flow of gas from the side of the tank exposed to the heat of the sun. The maximum pressure may be regulated by a pressure relief valve 48.

The sealing elements can be easily removed for repair and replaced without draining the tank. To remove a sectional element, all valves 41 are closed, and two sections of the pipe 44 are removed at the flanges of the T fittings 45 of the section to be removed and at adjacent sections, after which the openings at the adjacent sections may be closed by blanks and the other valves il again opened. The bolts 39 are removed, and the sectional sealing element lifted out of place.

I have found it to be desirable to construct all metal parts of corrosion-resistant material, such as aluminum.

Fig. illustrates a modined arrangement of the sectional sealing element, the modifications whereof may be employed separately or in combinaton in the arrangement previously disclosed. A plurality of compression springs 49 are mounted in the lower part of the compartment B, urging the sealing shoe and the medial wall 34C apart. This is desirable under certain conditions when the vapor pressure is insuicient to apply an even .pressure against the sealing shoe. A similar series of compression springs 5D ismounted in the compartment A; they may be used together with or in lieu of the liquid.

When the plate 3d is constructed of very llghtt weight material it may not have sufficient strength to support the shoe 33. In such case it is desirable to provide a separate support for the shoe. This is effected by means of a pair of horizontal beams, one for each end ofthe shoe. Oney of such beams is shown in Fig. 5 at 5l, secured to the sealing shoe 33 at 52, and passing beneath the rain gutter e2. The beam has a close it with this gutter to prevent the flow of water through the hole. The beam is slidably supported by means of angle bar 3 and 5d mounted on the frames El.

The beams El are, further, useful in connection with the use of the springs el?, not for supporting weight of the shoe 33, but for depressing it. The pressure of the vapors within the compartment B exert a considerable lifting force, which is, in the embodiments previously described, counteracted by the weight of the liquid in the compartment A. When, however, springs 56 are used in lieu oi the liquid, it is necessary to urge the shoe downward. This is achieved by the beam 5l.

Many modifications in the construction and arrangement of parts will be apparent to 'those skilled in the art without departing from the spirit and scope of the invention.

I claim as my invention:

l. in a floating roof tank, a sealing element comprising a curved sealing shoe engaging the wall of the tanl, a substantially horizontal, segmental shelf having its curved, outer portion secured and sealed to the inner tace oi the sealing shoe, a sealing sheet of resilient material joined directly to the inner chord oi said shelf to form a first straight hinge line at said chord and eX- tending downwardly therefrom to the roof and sealed thereto along a second straight hinge line, arranged so that the distance between the shoe and the roof may be varied by hinge-like movement about said hinge line, and means urging the upper part of the sealing sheet rotatively outwardly from the rooi' about said second hinge line, said shelf being made of resilient material affording suicient rigidity to transmit a horizontal thrust outwardly from said sealing sheet to the sealing shoe.

2. ln a floating roof tank, a sealing element comprising a sealing shoe engaging the tank wall and a compartmented pressure seal connecting said shoe and the roof and comprising a sealing sheet sealed to said shoe and roof shaped to pro vide a horizontal upright trough and a holontal inverted trough horizontally adjacent to said upright trough and having straight ge parallel to the length oi the troughs, anu yil able closure means at the ends of both troughs.

3. In a iioating roof tank, a sealing element comprising a sealing shoe engaging the tank wall and a compartmented pressure seal connect said shoe and the root, and comprising a sealing sheet of resilient material folded along straight lines to provide a horizontal upright trough and a horizontal inverted trough horizontally adjacent to and having a common wall with said upright trough and having hinge l'-I As along said folds, and yieldacle closure means at the ends of both troughs.

4. In a floating roof tank, a sealing element lcomprising a sealing shoe engaging the tank wall and a compartmented pressure seal connecting said shoe and the roof, and comprising a sealing sheet of durable material sealed to said shoe and roof shaped to provide a horizontal upright trough and a horizontal inverted trough horizontally adjacent to and having a common wall with said upright trough and having straight hinge lines parallel to the length of the troughs,

theends of b'oth troughs.

5. In a floating roof tank, the combination of a circular tank, a iioating roof, sectional sealing elements closing the space between the tank wall and the roof comprising sealing shoes engaging the tank wall and a compartmented pressure seal connecting said shoe and the roof containing a sealing sheet of substantially rigid material shaped to provide a horizontal upright trough and a horizontal inverted trough horizontally adjacent to said upright trough and having straight hinge lines parallel to the length of the troughs and yieldable closure means at the ends of the troughs, and a vapor pressure equalizer comprising conduit means connecting the inverted troughs of the several sectional sealing elements.

6. The combination according to claim in which the upright trough contains liquid, to a level to wet a substantial portion of the side walls of the trough whereby the liquid exerts an increasing pressure against the walls of the trough tending to urge said walls apart as the distance between said walls is decreased and produces a variable thrust tending to center the roof within the tank.

7. The combination according to claim 5 in which the upright trough contains compression springs arranged to urge the walls of the trough apart, thereby producing a variable thrust tending to center the roof within the tank.

8. In a oating roof tank, a sealing element comprising a curved sealing shoe engaging the tank wall and a pressure seal connecting the sealing shoe and the floating roof, said pressure seal comprising a substantially horizontal, segment-shaped shelf having its curved, outer portion secured and sealed to the inner face of the shoe, a first wall extending downwardly from the inner chord of said shelf and movable about a straight hinge line at said chord to form an inverted trough with said shelve, a second wall rising from a lower portion of said first wall and secured to the floating roof, said second wall being sealed to said iirst wall to form an upright trough therewith, said troughs being horizontally adjacent, said second wall being movable with respect to said rst wall about a straight hinge line to permit the distance between the walls of the troughs to change by movement about said hinge lines, and yieldable means closing the ends of both troughs.

9. The sealing element according to claim 8 in which the shelf and the iirst wall are made of the same piece of resilient, durable material.

10. In a floating roof tank, a sealing element comprising a sealing shoe engaging the wall of the tank, a compartmented pressure seal connecting said shoe and the roof comprising a sheet of resilient, durable material attached rigidly to the upper part of the shoe and folded downwardly to provide a downwardly extending wall and form an inverted trough between the shoe and the said downwardly extending wall, said sheet being further folded upwardly to provide an upwardly extending wall and form an upright trough between said downwardly and upwardly extending walls, said upwardly extending wall being sealed to the iioating roof, and corrugated bellows closing the ends of both troughs.

11. In combination with the/sealing element according to claim 10, a beam mounted on the floating roof secured to the sealing shoe to give vertical support thereto.

12. In combination with the sealing element according to claim 10, a pool of liquid in the upright trough, to a level to wet a substantial portion of the side walls of the trough whereby the liquid exerts an increasing pressure against the walls of the trough tending to urge said walls apart as the distance between said walls is decreased, and produces a variable thrust tending to center the roof within the tank.

1 3. In combination with the sealing element according to claim 10, compression springs in the upright trough arranged to urge said downwardly and upwardly extending walls apart, thereby producing a variable thrust tending to center the roof within the tank.

14. In a oating roof tank, the combination of contact means engaging the tank wall, an upright sectional trough positioned between the tank wall and a peripheral part of the roof, said trough having substantially rigid walls throughout, the side walls being sealed together at the bottom of the trough along a straight hinge line permitting the upper parts of the side walls to move apart, said walls having connections above their bottom portions, one to the roof and the other to said contact means, and at least one of said connections being along a straight hinge line parallel to said hinge line at the bottom of the trough, whereby the upper portions of said walls are movable towards and away from each other by movement about said parallel hinge lines and are arranged to exert an outward thrust from the roof to said contact means when the said walls are forced apart, and means urging said walls apart with a force which increases as the distance between said walls is decreased.

15. The combination according to claim 14 in which the said means urging the walls apart comprises a body of liquid in said trough having a level to wet a substantial portion of the side walls of the trough, whereby the liquid exerts an increasing thrust against the walls of the trough as the distance between said walls is decreased.

ALEXANDER S. FEILD.

REFERENCES CITED The following references are of record in the lle of this patent:

UNITED STATES PATENTS Number Name Date 1,426,998 Leland et al Aug. 22, 1922 1,592,244 Wiggins July 13, 1926 1,775,758 George Sept. 16, 1930 1,854,535 Wiggins Apr. 19, 1932 1,930,953 Hampton Oct. 17, 1933 2,244,782 Jones June 10, 1941 2,282,772 Wiggins May 12, 1942 2,314,805 Wiggins Mar. 23, 1943 

